1. Field of the Invention
This invention relates to a method of compressing image signals. This invention particularly relates to a method of compressing image signals so that a high signal compressibility is obtained by utilizing block encoding and vector quantization.
2. Description of the Prior Art
Image signals representing half tone images, such as television signals, are composed of enormous amounts of information, and a broad-band transmission line is required for transmission of the image signals. Such image signals involve much redundancy, and various attempts have been made to compress the image signals by restricting the redundancy. Also, in recent years, recording of half tone images on optical disks, magnetic disks, or the like has been generally put into practice. In this case, image signal compression is conducted generally for the purpose of efficiently recording image signals on a recording medium.
One of the methods of image signal compression that has heretofore been known is a method wherein block encoding is utilized. The method of compressing image signals by block encoding comprises the steps of (i) dividing the original image signals representing a two-dimensional image into blocks each of which is composed of a plurality of samples, (ii) converting the original image signals at the respective picture elements in each of the blocks into n-valued system where n denotes an integer of 2 or larger, (iii) calculating means values m.sub.1, m.sub.2, . . . , m.sub.n of the original image signals at the respective picture elements in the block that have the common n-valued signals, and (iv) encoding the mean values m.sub.1, m.sub.2, . . . , m.sub.n and the n-valued signals.
With the aforesaid method of compressing image signals by block encoding, in the case where the density scale of a single picture element is of, for example, 256 levels (=8 bits), division into blocks is carried out in the unit of 4.times.4=16 picture elements and the original image signals are converted into the two-valued system, the amount of the image signals per block becomes equal to 16.times.1 bits plus the amount of signals representing the mean values m.sub.1 and m.sub.2 (both may be of approximately 8 bits). Thus the image signals can be compressed substantially as compared with the original signal amount (=16.times.8 bits).
Ordinarily, the image signals in the block have high correlation therebetween. Therefore, in the case where the aforesaid mean value is employed for the picture elements whose n-valued signals are equal to one another, the mean value corresponds to the original image signals with a slight distortion. Accordingly, in the course of image reconstruction, the aforesaid mean values are employed for the respective picture elements on the basis of the n-valued signals at the respective picture elements, and the image is reproduced on the basis of the signals. In this manner, a reconstructed image which is not so different from the original image can be obtained.
The aforesaid method of compressing image signals utilizing block encoding gives rise to little deterioration in the image quality in the case where the image composed of many low-frequency components, and therefore is suitable particularly for a radiation image composed of many low-frequency components. However, a need exists for further improvement of the signal compressibility.